Freeze-dried resorcinol-formaldehyde gels

Mathieu, B.; Blacher, Silvia; Pirard, René; Pirard, Jean-Paul; Sahouli, Bendida; Brouers, F.

doi:10.1016/s0022-3093(97)00025-2

Cited by 59 publications

(37 citation statements)

References 19 publications

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“…The corresponding carbon materials develop relatively high specific surfaces S BET ranging from 430 to 630 m²/g and total void volumes V v varying from 0.3 to 2.4 cm³/g. This is similar to results obtained with more sophisticated techniques [2], [3], [12]. It must be pointed out that the apparent density obtained at pH = 6 (0.35 g/ cm³) corresponds exactly to the theoretical density when no shrinkage occurs during drying, as observed in this paper.…”

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confidence: 90%

“…Supercritical conditions suppress the liquid-vapor interface, avoiding shrinkage and cracking of the material during solvent removal and preserving the porous texture. As supercritical drying remains difficult to apply at an industrial scale because of its expensive and potentially dangerous character, other softer drying techniques have been tested in order to produce an aerogel-like mesoporous texture: freeze-drying [2], vacuum drying [3], microwave drying [4], solvent exchange followed by freeze drying [5] or drying under nitrogen in tube furnace [6],... Quite surprisingly, it appears that conventional convective drying, with controlled air temperature, velocity and humidity, has never been used in order to produce RF xerogels. As this technique is well known and largely used in the industry, it seemed interesting to study its suitability: would this technique enable us to obtain porous RF (and carbon after pyrolysis) xerogels?…”

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confidence: 99%

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Suitability of convective air drying for the production of porous resorcinol–formaldehyde and carbon xerogels

Léonard

Job

Blacher

et al. 2005

Carbon

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Production of carbon aerogels by CO 2 supercritical drying of resorcinol-formaldehyde (RF) hydrogels followed by pyrolysis in inert atmosphere has been extensively described in the scientific literature, since their introduction by Pekala in 1989 [1]. Supercritical conditions suppress the liquid-vapor interface, avoiding shrinkage and cracking of the material during solvent removal and preserving the porous texture. As supercritical drying remains difficult to apply at an industrial scale because of its expensive and potentially dangerous character, other softer drying techniques have been tested in order to produce an aerogel-like mesoporous texture: freeze-drying [2], vacuum drying [3], microwave drying [4], solvent exchange followed by freeze drying [5] or drying under nitrogen in tube furnace [6],... Quite surprisingly, it appears that conventional convective drying, with controlled air temperature, velocity and humidity, has never been used in order to produce RF xerogels. As this technique is well known and largely used in the industry, it seemed interesting to study its suitability: would this technique enable us to obtain porous RF (and carbon after pyrolysis) xerogels?Hydrogels have been prepared following the method described by Job et al. [3]. The molar ratio R/F and the dilution ratio D, as defined by these latter, were fixed at 0.5 and 5.7 respectively. Three initial values of the pH of the precursors solution were chosen: 6. 6.5 and 7 (±0.05). Cylindrical samples were obtained by casting 5 ml solution into sealed glass moulds (∅ = 22 mm) and putting them for gelation in an oven at 85°C during 72 h. After gelation, the samples had a mass comprised between 4.2 and 5.1 g. They have been dried in a classical convective rig, with air at ambient humidity, at a temperature of 70°C and a superficial velocity of 2 m/s, i.e. quite severe drying conditions. Fig. 1a shows the drying curves, i.e. the evolution of mass with time, for the three samples. In each case, mass stabilization occurs after approximately 4 to 5 hours, indicating the end of the drying process. In comparison with vacuum drying, the drying duration is about 40 times shorter [3]. The three samples reached a final mass close to 1.45 g and kept their monolithic form.Volumetric shrinkage was negligible when pH = 6 (orange xerogel), reached 21% when pH = 6.5 (light brown xerogel) and 60% when pH = 7 (dark brown xerogel). At high pH, a large shrinkage is observed even with supercritical drying, because of the 'polymeric' character of the gel [7]. The drying kinetics are represented in Fig. 1b by plotting the drying flux (kg/m²s) vs. the water content W (kg/kg), expressed on a dry basis, as commonly encountered in the drying literature. A

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confidence: 90%

mentioning

confidence: 99%

Suitability of convective air drying for the production of porous resorcinol–formaldehyde and carbon xerogels

Léonard

Job

Blacher

et al. 2005

Carbon

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“…As supercritical drying remains difficult to apply at an industrial scale regarding its expensive and potentially dangerous character, other softer drying techniques have been tested in order to produce an aerogel-like mesoporous texture: freeze-drying [12], vacuum drying [13], microwave drying [14], solvent exchange followed by freeze drying [15] or drying under nitrogen in tube furnace [16]. Some of us have shown recently that it is possible to produce porous resorcinol-formaldehyde xerogels by using atmospheric convective drying to remove the solvent, without any preliminary treatment [17].…”

Section: Introductionmentioning

confidence: 99%

Evolution of mechanical properties and final textural properties of resorcinol–formaldehyde xerogels during ambient air drying

Léonard

Blacher

Crine

et al. 2008

Journal of Non-Crystalline Solids

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Porous carbon xerogels can be obtained by convective drying of resorcinol (R)-formaldehyde (F) hydrogels, followed by pyrolysis. Drying conditions have to be carefully controlled when crack-free monoliths with well-defined shape and size are required. The knowledge of the mechanical properties of the RF xerogels and their evolution with water content is essential to model their thermo-hygro-mechanical behaviour during convective drying and avoid mechanical stresses leading to deformation and cracking of the sample. The shrinkage behaviour and the mechanical properties of RF xerogels obtained with R/C ratio ranging from 300 to 1500 were investigated. R/C greatly influences the shrinkage and mechanical properties of the wet gel, on the one hand, and the mechanical and textural properties of the dried gel, on the other hand. The smaller the R/C, the higher the shrinkage, the stiffening, and the viscoelastic character of the xerogels. Water content has an influence on both the stiffness of the gels and the viscoelastic response. Generally, samples lose their mechanical viscous character and become more rigid when they are dried. Finally, mercury porosimetry measurements showed that the gels exhibit a marked lowering of 2 their stiffness upon compression, interpreted as a result of the heterogeneity of the microstructure.

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“…The gels can be directly synthesized in acetone [9] or other solvents to bypass the solvent exchange step, but since water is produced during the polymerisation reaction, surface tensions problems are not completely avoided during supercritical drying, which sometimes induces shrinkage. Freezedrying was envisaged as an alternative to supercritical drying [10][11][12], but the pore texture obtained is often heterogeneous, especially in the case of low-density materials, and monolithicity cannot be easily maintained. A few studies were conducted using subcritical drying performed on resorcinol-formaldehyde gels after solvent exchange (water to acetone) in order to minimize the shrinkage due to surface tensions [13][14][15].…”

Section: Introductionmentioning

confidence: 99%

Synthesis optimization of organic xerogels produced from convective air-drying of resorcinol–formaldehyde gels

Job

Panariello

Marien

et al. 2006

Journal of Non-Crystalline Solids

110

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Resorcinol-formaldehyde gels were produced at 50, 70 and 90°C and with three different R/C ratios (500, 1000 and 2000). The effect of these variables combined with that of aging time was studied in order to optimize the synthesis conditions. The convective air-drying process was used, and the drying duration was studied with regard to the synthesis conditions. The aging time has no effect on the pore texture after 24 h at 90°C or 48 h at 70°C, whatever the R/C value. The synthesis-aging step can be shortened by increasing the temperature.Nevertheless, the pore size tends then to decrease, especially when R/C is high, but this can be counterbalanced by increasing R/C. Moreover, bubbles often appear in the gel at high synthesis temperature, which limits the temperature to about 70°C in the case of monolithic parts. At 70°C and with an air velocity of 2 m/s, the elimination of 90% of the solvent Manuscript 2 requires 1 h drying when the pore size reaches 400-600 nm, 2.5 h for 50 nm wide pores and 3 h when the pore size decreases to 15-20 nm. The drying duration does not exceed 8 h in all cases and could be shortened by increasing the temperature at the end of the process.

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Freeze-dried resorcinol-formaldehyde gels

Cited by 59 publications

References 19 publications

Suitability of convective air drying for the production of porous resorcinol–formaldehyde and carbon xerogels

Suitability of convective air drying for the production of porous resorcinol–formaldehyde and carbon xerogels

Evolution of mechanical properties and final textural properties of resorcinol–formaldehyde xerogels during ambient air drying

Synthesis optimization of organic xerogels produced from convective air-drying of resorcinol–formaldehyde gels

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